This invention relates generally to solid-state circuit interrupters in electrical distribution systems and more particularly, it relates to an improved microprocessor watch-dog monitor for electronic trip units used in circuit interrupters so as to maintain the integrity of the trip units.
As is generally well-known in the art, circuit breakers have been widely used in commercial and industrial applications for protecting electrical conductors and apparatus connected thereto from damage due to excessive current flow. Circuit breakers typically included trip systems which were designed to interrupt when the current flowing through them exceeded a predetermined level. Specifically, most simple trip systems utilized an electromagnet to trip the circuit in response to current or voltage fluctuations. The electromagnet provided a magnetic field in response to current flowing through the circuit breaker. When the current level increased beyond the predetermined level or trip point, the magnetic field "trips" a mechanism which causes a set of circuit breaker contacts to release, thereby "opening" or "breaking" the circuit path.
Gradually, however, there has arisen a need in the industry for more sophisticated and elaborate tripping systems as the complexity of electrical distribution systems increased. For example, in many commercial and industrial equipment today it is desired to have circuit breakers that perform both an instantaneous and delayed tripping (i.e., time-current interrupting characteristics) so as to provide improved accuracy and flexibility on the equipment to be controlled. For this reason, many microprocessor-based solid-state circuit interrupters have been also developed in the prior art in an attempt to provide more accurate and reliable control operations on the electrical distribution system on which the circuit interrupter was being employed. To this end, a microcomputer is provided which is coupled between the current path and a trip solenoid controlling the mechanism for breaking the current path. The microcomputer stores trip points which activate the trip solenoid when the current within the current path exceeds the trip points.
Therefore, in order to enhance system dependability there is generally required a watch-dog circuit for protecting the tripping system in the event of a microcomputer malfunction. Prior art watch-dog circuits are known to exist which include either an operational amplifier or a discrete programmable unijunction transistor. While these prior art circuits performed their function adequately, they suffer from the disadvantage of being high in cost and having a low reliability.
Accordingly, the present invention is directed to an improved microprocessor watch-dog monitor which provides for more reliable operation and performance at a reduced cost. Specifically, the microprocessor watch-dog monitor of the instant invention includes a comparator circuit formed of a first bipolar transistor of the PNP-type conductivity and a second bipolar transistor of the NPN-type conductivity.